Among the metals obtained in electrolytic cells operating at high temperature in a molten salt electrolyte containing an oxide or compound of the metal to be electrowon, aluminium is the most important and the invention will describe in particular the protection of components of aluminium cells, more particularly the protection of the cell cathode bottom by applying an aluminium-wettable, adherent coating.
Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in molten salt containing cryolite at temperatures around 950.degree. C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar.
In Hall-Heroult cells, a molten aluminium pool acts as the cathode. The carbon lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode beneath the aluminium pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminium through the carbon body and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to displacement of the cathode blocks, aluminium reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact and an excessive iron content in the aluminium metal produced.
A major drawback of carbon as cathode material is that it is not wetted by aluminium. This necessitates maintaining a deep pool of aluminium (at least 100-250 mm thick) in order to ensure a certain protection of the carbon blocks and an effective contact over the cathode surface. But electromagnetic forces create waves in the molten aluminium and, to avoid short circuiting with the anode, the anode-to-cathode distance (ACD) must be kept at a safe minimum value, usually 40 to 60 mm. For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting between the aluminium pool and the anode. The electrical resistance of the electrolyte in the inter-electrode gap causes a voltage drop from 1.8 to 2.7 volts, which represents from 40 to 60 percent of the total voltage drop, and is the largest single component of the voltage drop in a given cell.
To reduce the ACD and associated voltage drop, extensive research has been carried out with Refractory Hard Metals (RHM) such as TiB.sub.2 as cathode materials. TiB.sub.2 and other RHM's are practically insoluble in aluminium, have a low electrical resistance, and are wetted by aluminium. This should allow aluminium to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminium pool. Because titanium diboride and similar Refractory Hard Metals are wettable by aluminium, resistant to the corrosive environment of an aluminium production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present many advantages, notably including the saving of energy by reducing the ACD.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminium production cells is described in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminium industry.
The non-acceptance of tiles and other methods of applying layers of TiB.sub.2 and other RHM materials on the surface of aluminium production cells is due to their lack of stability in the operating conditions, in addition to their cost. The failure of these materials is associated with penetration of the electrolyte when not perfectly wetted by aluminium, and attack by aluminium because of impurities in the RHM structure. In RHM pieces such as tiles, oxygen impurities tend to segregate along grain boundaries leading to rapid attack by aluminium metal and/or by cryolite. To combat disintegration, it has been proposed to use highly pure TiB.sub.2 powder to make materials containing less than 50 ppm oxygen. Such fabrication further increases the cost of the already-expensive materials. No cell utilizing TiB.sub.2 tiles as cathode is known to have operated for long periods without loss of adhesion of the tiles, or their disintegration. Other reasons for failure of RHM tiles have been the lack of mechanical strength and resistance to thermal shock.
Various types of TiB.sub.2 or RHM layers applied to carbon substrates have failed due to poor adherence and to differences in thermal expansion coefficients between the titanium diboride material and the carbon cathode block.
U.S. Pat. No. 3,400,061 describes a cell without an aluminium pool but with a drained cathode of Refractory Hard Metal which consists of a mixture of Refractory Hard Metal, at least 5 percent carbon, and 10 to 20% by weight of pitch binder, baked at 900.degree. C. or more and rammed into place in the cell bottom. Such composite cathodes have found no commercial use probably due to susceptibility to attack by the electrolytic bath.
U.S. Pat. No. 4,093,524 discloses bonding tiles of titanium diboride and other Refractory Hard Metals to a conductive substrate such as graphite. But large differences in thermal expansion coefficients between the RHM tiles and the substrate cause problems.
U.S. Pat. No. 3,661,736 claims a composite drained cathode for an aluminium production cell, comprising particles or pieces of arc-melted "RHM alloy" embedded in an electrically conductive matrix of carbon or graphite and a particulate filler such as aluminium carbide, titanium carbide or titanium nitride. However, in operation, grain boundaries and the carbon or graphite matrix are attacked by electrolyte and/or aluminium, leading to rapid destruction of the cathode.
U.S. Pat. No. 4,308,114 discloses a cathode surface of RHM in a graphitic matrix made by mixing the RHM with a pitch binder and graphitizating at 2350.degree. C. or above. Such cathodes are subject to early failure due to rapid ablation, and possible intercalation by sodium and erosion of the graphite matrix.
To avoid the problems encountered with tiles and with the previous coating methods, U.S. Pat. No. 4,466,996 proposed applying a coating composition comprising a preformed particulate RHM, such as TiB.sub.2, a thermosetting binder, a carbonaceous filler and carbonaceous additives to a carbonaceous cathode substrate, followed by curing and carbonisation. But it is still not possible by this method to obtain coatings of satisfactory adherence that could withstand the operating conditions in an aluminium production cell. It has also proven impossible to produce adherent coatings of RHM on refractory substrates such as alumina.
U.S. Pat. No. 4,560,448 describes a structural component of an aluminium production cell which is in contact with molten aluminium, made of a non-wettable material such as alumina which is rendered wettable by a thin layer (up to 100 micrometer) of TiB.sub.2. However, to prevent dissolution of this TiB.sub.2 layer, the molten aluminium had to be maintained saturated with titanium and boron and this expedient was not acceptable.
U.S. Pat. No. 5,004,524 discloses a body of fused alumina or another refractory oxycompound having a multiplicity of discrete inclusions of TiB.sub.2 or other aluminium-wettable RHM cast into its surface. This material is particularly suitable for non-current carrying cathode bottom floors of aluminium production cells, but in the long term even if the material may remain bound to the fused alumina and resist to corrosion, the manufacture at an acceptable cost remains a problem.
U.S. Pat. No. 4,595,545 discloses the production of titanium diboride or a mixture thereof with a carbide and/or a nitride of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum or tungsten by carbothermic, carbo-aluminothermic or alumino-thermic reaction, under vacuum or an inert atmosphere, of a glass or microcristalline gel of oxide reactants prepared from organic alkoxide precursors. This glass or gel was then ground and formed into bodies and sintered into bodies of titanium diboride/alumina-based materials as components of aluminium production cells. But such sintered materials are subject to attack and grain-boundary corrosion when in contact with molten aluminium. Similar reactions, known as combustion synthesis, self-propagating high temperature synthesis or micropyretic reactions are known (see below, under the heading "Micropyretic Reactions"), but to date these reactions have not been applied to the production of refractory coatings on carbonaceous, refractory or other substrates in such a way, and with the right composition, as no lead to coatings with adequate adherence to survive the operating conditions in an aluminium production cell.
U.S. Pat. No. 4,600,481 proposed making components of aluminium production cells by infiltrating aluminium into a skeletal self-sustaining matrix of alumina or another refractory material which is normally non-wettable by molten aluminium, after having rendered the surface of the matrix wettable by molten aluminium for instance by treating the surface with a wetting agent such as titanium diboride, in particular with a titanium diboride composite material produced according to the previously-mentioned patent. In this case, only a temporary surface wetting was thought to be required to facilitate the infiltration, but in practice it was not easy to produce materials that sufficiently maintained the internal wetting to sustain long operating periods when the component was exposed externally to molten aluminium. Also, the described techniques have not been applied to external surfaces of refractory bodies to make them permanently wettable by molten aluminium.
The methods employed to date have thus not successfully produced adherent protective coatings of refractory materials, in particular aluminium-wettable refractory materials such as TiB.sub.2 and other Refractory Hard Metals, on various substrates and in particular on carbonaceous or refractory substrates, that adhere to and remain firmly attached to the substrate in conditions such as encountered in aluminium production cells, the coating providing a permanent and perfectly protective surface that is wetted by molten aluminium.